Fix: Install Apple Silicon shows Universal as Installing

[William-Laverty]

Summary

When clicking 'Install Apple Silicon', the UI incorrectly shows the Universal version as 'Installing' instead of the Apple Silicon version.

Root Cause

setInstallationStep used version.isEquivalent() which doesn't consider architectures. When both Universal and Apple Silicon versions exist with the same Xcode version, the first one found (Universal) was marked as installing.

// Before: Uses version equivalence (ignores architectures)
func setInstallationStep(of version: Version, to step: XcodeInstallationStep) {
    guard let index = self.allXcodes.firstIndex(where: { $0.version.isEquivalent(to: version) }) else { return }
    // ...
}

Fix

Changed setInstallationStep to accept XcodeID instead of Version, enabling exact matching including architectures:

// After: Uses XcodeID equality (includes architectures)
func setInstallationStep(of xcodeID: XcodeID, to step: XcodeInstallationStep) {
    guard let index = self.allXcodes.firstIndex(where: { $0.id == xcodeID }) else { return }
    // ...
}

Testing

1. Filter list to show only Apple Silicon builds
2. Select an Xcode version with both Universal and Apple Silicon variants
3. Click 'Install Apple Silicon'
4. Verify the Apple Silicon row shows as 'Installing', not the Universal row

Fixes #776, #777, #782